A developmental psychopathology approach to Borderline Personality Disorder (BPD) is needed to elucidate our understanding of this complex and debilitating disorder. The multifaceted nature of dysfunction in BPD makes the disorder especially difficult to empirically research and clinically treat. There is a need to identify features that might contribute to the etiology of the disorder to ultimately aid prevention and intervention efforts. AI is a core BPD feature that drives the development of other features including cognitive disturbances, impulsive behaviors, identity confusion, and interpersonal dysfunction 1-3. By examining the processes of affect regulation in the context of normal development, we can gain insights into processes that go awry and lead to AI. I have developed hypotheses about affect regulation processes on the pathway to BPD in adolescent girls. The rapid changes in social expectations, biology, and hormones during adolescence increase the demands on affect regulation skills. Thus, adolescence is an optimal time to examine affect regulation in the manifestation of BPD features. Specifically, girls with high levels of AI will develop other BPD features (impulsivity, interpersonal dysfunction, and identity disturbances). The training and research plans described in this proposal are designed to enable me to test the validity of these claims. Contextually relevant paradigms and improved methodologies going beyond self-report questionnaires are needed to examine AI and its developmental consequences. In the proposed study, I will examine both the pupil dilation patterns of adolescent girls at high and low levels of AI and face-to-face interactions with their mothers. These methodologies allow for both the psychophysiological and observational assessment of AI. This proposal will advance our understanding of the developmental associations of BPD, a costly and significant mental health problem4,5. This work will address the NIMH strategic objective of promoting "Discovery in the Brain and Behavioral Sciences to Fuel Research on the Causes of Mental Disorders." PUBLIC HEALTH RELEVANCE: Borderline Personality Disorder (BPD) is a devastating and complex psychiatric condition affecting nearly 2 million adults annually that is associated with several negative outcomes, including social stigma, poor treatment response, and poor social, occupational, and academic outcomes 6-10. There is a critical need for research that addresses the developmental course of BPD symptoms. The proposed study seeks to examine the role of affect instability(AI) in the development of borderline personality features in adolescent girls using multiple assessment methods, resulting in improved intervention efforts that target AI and may have a higher impact during adolescence than adulthood.